1. Field of the Invention
The present invention relates to a nonreciprocal circuit device, such as a lumped-constant isolator or circulator, for use in the microwave band.
2. Description of the Related Art
Recently there has been a trend in the field of mobile communications or portable telephony toward the use of equipment using digital modulation techniques such as 1/4-pi QPSK or CDMA, which provide more efficient use of bandwidth. As shown in FIG. 9, such digital communications equipment uses a linear power amplifier 20 as the transmitting power amplifier. This has a structure in which an input matching circuit 21, a first-stage amplifier 22, an interstage matching circuit 23, a second-stage amplifier 24 and an output matching circuit 25 are arranged and connected.
When a linear amplifier 20 of this type is used, the power consumption of the power amplifier has a major effect on the length of the talk-time of a battery-operated portable telephone set. Thus, a reduction in power consumption can make a dramatic improvement in efficiency.
It is, however, a characteristic of the above-described high-efficiency linear amplifiers that they are highly subject to changes in load impedance. That is to say, high-efficiency amplification is achieved only when load impedance is constant at a desirable value. For example, when a load, such as an antenna, is directly connected to a linear amplifier as described above, and its input impedance undergoes major changes, problems of decreasing amplification efficiency and degradation of input/output linearity occur. This may result in an increase in the power consumption of the transmitting power amplifier, thereby draining the battery and reducing talk-time, or in distortion of the transmission signal, thereby producing interference in adjacent channels or on adjacent frequencies. There is even the danger that modulation distortion could prevent modulation at the receiving set, rendering transmission impossible.
To overcome this problem, a lumped-constant isolator 27 may be inserted between linear amplifier 20 and antenna 26. This isolator has a structure, as shown in FIG. 8, having three center electrodes 30 through 32 disposed so as to intersect each other with given intervals therebetween, and having a ferrite 33 disposed in alignment with the point of intersection, a DC magnetic field HDC being applied, and terminal resistor R being connected to port P3 of center electrode 32.
Since the input impedance of isolator 27 is stable irrespective of changes in load impedance, it has the function of absorbing reflected energy from the antenna, thereby improving the matched state. By this means, decreases in the efficiency of the linear amplifier and degradation of input/output linearity are prevented.
Further, the input/output impedance of linear amplifier 20 is generally set at 50 ohms and the input impedance of isolator 27 is also generally set at 50 ohms, and this constitutes a standard for radio-frequency components.
On the other hand, with decreases in the size and weight of such portable telephone sets has come progress in simplifying the structure of batteries, so that in some cases voltages are now set in the vicinity of 3.6 to 6 V. In order to allow the linear amplifier to continue operating if the battery voltage falls below 3.6 V, for example, the power supply voltage of the linear amplifier may be set at approximately 3.0 V.
Furthermore, the saturation power (that is to say the power at which an increase in input produces no further increase in output) of such linear amplifiers is determined by the power supply voltage and the output impedance of the amplifier device (transistor, field-effect transistor, or more recently GaAs FET). Thus in order to obtain some margin in terms of saturation power, it is usual for the saturation power of a linear amplifier with a rated output power of, say, 1 W, to be set at 2 W or thereabouts.
It should be noted, however, that as shown in FIG. 9, at such a low power supply voltage, the output impedance Zo of output amplifier 24 will be 2 to 6 ohms, far lower than the output impedance of a linear amplifier set at the usual value of 50 ohms. To convert such a low impedance to 50 ohms, it is necessary to provide an output matching circuit 25 having a large impedance conversion ratio, but this brings about an increase in losses in the conversion circuitry and a narrowing of the frequency range within which satisfactory matching can be attained. This results in the problem of degradation in the efficiency of the power amplifier and in the operating frequency bandwidth.